DIFFERENTIAL INDUCTION OF ETOPOSIDE-MEDIATED APOPTOSIS IN HUMAN LEUKEMIA HL-60 AND K562 CELLS

Etoposide (VP-16) is one of several DNA-damaging agents that induce su bcellular structural changes associated with apoptosis. VP-16 exerts i ts DNA-damaging and cytotoxic effects subsequent to interference with DNA topoisomerase II activity. VP-16 also stimulates c-jun and c-fos m RNA expression in some cell lines, including human leukemia K562 and H L-60 cells. To compare the temporal relationship between drug-induced c-jun expression and apoptosis, we examined cell morphology, cell viab ility, DNA integrity, and c-jun induction during VP-16 treatment of K5 62 and HL-60 cells. VP-16 (10 mu M)-induced internucleosomal DNA damag e and nuclear fragmentation were readily apparent within 6 hr in HL-60 cells but were absent in K562 cells treated for up to 24 hr. Some int ernucleosomal DNA damage was observed in K562 cells but only after tre atment with 100 mu M VP-16 for 24 hr. In contrast, VP-16-induced DNA s ingle-strand breaks, VP-16-induced topoisomerase II/DNA covalent compl ex formation, and VP-16-mediated growth inhibition were similar in K56 2 and HL-60 cells. Also, the time course of VP-16-induced c-jun mRNA e xpression was comparable for both K562 and HL-60 cell lines. Western b lot analysis of whole-cell lysates showed that Bcl-2 protein levels we re 13-fold greater in HL-60 cells than in K562 cells. Thus, the resist ance of VP-16-treated K562 cells to apoptosis was not attributable to protection by Bcl-2. Furthermore, the relatively high levels of Bcl-2 in HL-60 cells were not sufficient to protect these cells against apop tosis. Together, our results indicate that the temporal coupling of VP -16-induced DNA damage, c-jun expression, and apoptosis is cell type s pecific and suggest that different signaling pathways for apoptosis ar e operating in these two human leukemia cell lines.